Apparatus for drying



APPARATUS FOR DRYING Original Filed July 26, 1939 INVENTOR. FRANCIS. E.fi/CWOWSZV ATTORNEYS Patented July 15, 1941 I 2,249,625 APPARATUS ronDRYING Francis R. Bichowsky, Ann Arbor, Mich., assignor to The DowChemical Company, Midland,

Mich., a corporation of Michigan Original application July 26, 1939,Serial No.

Divided and this application November 10, 1939, Serial No. 303,920

8 Claims.

This invention relates to apparatus for drying materials. Moreparticularly, it concerns improved apparatus for removing moisture frommaterials by bringing into contact therewith a relatively dry atmosphereof controlled temperature and humidity, this atmosphere beingsubsequently conditioned for re-use by contact with a hygroscopicliquid.

Inasmuch as the hygroscopic solution in the liquid circulating systembecomes diluted during operation, a small portion of the solution iswith- 1 drawn from the line It through a valve .I1 and One of theobjects of the invention is to provide highly eiiicient apparatus fordrying moist materials. Another object is to describe apparatus forconducting drying operations under conditions which are substantiallyadiabatic and largely independent of the surrounding atmosphere. Stillanother object is to provide an adequate system of controls for suchapparatus.

with these and other objects in view, the apparatus of the presentinvention includes means for passing a stream of air of predeterminedtemperature and relative humidity in a closed system into contact withmaterials to be dried and directly thereafter into contact with ahygroscopic solution of controlled temperature and concentration,whereby the air is brought to the proper condition for recirculationwithout having to be rejected or replaced in part with fresh air. "Thiscontinued re-use of the same air permits large heat savings.

The invention may best be understood with reference to the accompanyingdrawing which illustrates diagrammatically a preferred embodiment of theinvention.

In the'apparatus illustrated, drying is conducted in a chamber I withshelves 2 or holding the material to be dried. Air is circulated throughthe drier in a closed circuit comprising a motor-driven air-circulatingfan 3, an air delivery duct 4 connecting the fan to the chamoer I, andan outlet duct 5 connected through a damper 6 to the air-conditioningchamber I and through a damper 8 to the by-pass 9, the

is conveyed through a line I8 and a. heat-exchanger jacket l9 into anevaporator or regenerator 20 which is fired by a gas burner 2|controlled by a gas valve 22. The reconcentrated liquor from the bottomof the regenerator is then returned to the liquid circulating systemthrough a heat-exchanger coil 23, or a by-pass 24 controlled by a valve25, and thence through a return line 26 leading to the sump l2. Thisline 26 is provided with a small heat exchanger 21 to which coolingwater may be admitted as needed through a valve 28.

'I'he concentration of the hygroscopic solution in the apparatus may becontrolled by any desired means, suitably by varying the rate ofevaporation in the regenerator 20, e. g. by adjusting the gas-flow tothe burner 2|. This control may be effected automatically by means ofopposite end of both the chamber land the byliquid circulating systemcomprising pumpjl3 and connecting piping, are preferably provided withinsulating lagging (not shown) to preventtrans- I mission of heat intoor out of the system, i

a density indicator, 29 receiving hygroscopic liquid through a by-pass30, the density-responsive plummet 3| oi the device being connected tothe burner valve22 (as shown'by the solid line) in such manner that thegas flow to the burner is varied in accordance with the indications ofthe plummet 3|. The'temperature and humidity of the air circulatedthrough the driers are regulated by a thermostat 32 and a wet-bulbthermometer 33 locatedwithln the drying chamber I, the thermostat 32being adapted to actuate the valve 16 controlling the rate ofcirculation of the hygroscopic liquid, and the wet-bulb thermostat 33being operatively connected to valve 25 controlling the by-pass 24 (asshown by the full line) and simultaneously or alternatively to the valve28 controlling the cooler 21 (as shown by the broken line). i l

r In operating the apparatus according to the invention, the materialsto be dried are placed upon the shelves 2 or otherwise conveyed into thedrying chamber, and the 'fan 3 is set in motion causing the air in the.system to circulate through the-chamber I in contact with the materialto be dried. In this chamber, since in operation the partial pressure ofwater in the air is less than-the vapor pressure of the moisture in thematerial being dried, this moisture will evaporate into the air stream,such evaporation cooling the air and increasing itshumidity. In asmuchas the'chamber i is thermally insulated, this evaporation and coolingoccurs under es'sentially adiabatic conditions, and the air remains-tion of air.

at substantially constant total heat, i. e. at constant wet-bulbtemperature.

The cooled moist air leaving the chamber i passes through the duct anddamper 6 into the conditioning chamber 1, the by-pass damper 8 beingclosed in this mode of operating. In the chamber 1 the moisture-ladenair is brought into contact with a hygroscopic liquid circulated by thepump l3, either in the form of a shower or spray, or in distributed formupon a suitable medium such as cotton strings suspended from thereservoir II, as shown. In this air-liquid contact zone the hygroscopicliquid absorbs part of, usually most of, the moisture from the air, suchabsorption being accompanied by a liberation of the heat of condensationand solution of the Water vapor, this heat raising the temperature ofthe hygroscopic liquid, which, in turn, transfers a part of this heatback to the air in contact therewith. .The air, thus warmed and dried,is ready for recycling in the process through the duct Hi to the inletof the fan 3. As operation is continued, the hygroscopic liquid'beingcirculated into .contact with moist air in the chamber 1 continues toheat p, as explained, until it reaches and remains at some temperatureabove that of the incoming air, at which temperature the amount of heattransferred from the liquid to the air is exactly equal to the heatliberated in the dehumidifics Now, since thechamber I isthermallyinsulated and since, as will be explained, the hygroscopic liquidcirculating through the reservoir H, sump l2, pump l3, and pipes I4 andI5 does not change in temperature appreciably and is maintained atconstant composition, it is seen that the heating and dehumidificationof the air occurring in the chamber 1 takes place under adiabaticconditions, i. e. the air remains at constant heat content and hence ata constant wet-bulb temperature.

As hereinbefore explained, the hygroscopic liquid being circulatedthrough the chamber I is maintained at constant concentration bywithdrawing a small portion, e. g. 5-10 per cent of the total, andremoving water therefrom, as in the evaporator 20, which is controlledby the density-responsive plummet 3|. The reconcentrated liquor returnedfrom the evaporator 21! through the pipe 26 should be at substantiallythe same temperature as the diluted portion withdrawn through line 18 inorder to avoid adding or subtracting heat from the otherwise adiabaticcirculatory system for the hygroscopic liquid. This necessarytemperature control may be effected by any desired means, as byoperating the evaporator 20 under such a vacuum that the water isremovedwithout heating the liquor. Al

ternatively, the evaporator 20 may be. run hot at atmospheric pressure,the incoming diluted liquor being used to cool the reconcentrated liquorback to the temperature of the circulating liquor, in a heat-exchanger23, as illustrated. If the heatcxchanger 23, by reason of inefiiciency,does not provide enough cooling, a further temperature reduction may beeffected by running cooling water through the small heat-exchanger 21.

Although drying operations may be carried out satisfactorily in theapparatus as thus far described, it has been found that unless means areemployed to control the air-reconditioning step in the chamber 1 thereis no assurance that the amount of water absorbed by the h roscopicsolution will precisely equal the amount of Water given off by thematerial being dried. If a condition of inequality prevails, thetemperature and humidity of the air passing into the drying chamber Iwill not remain constant, but will vary somewhat as the drying proceeds.In certain instances this variation is of no great consequence, but fordrying numerous materials, especially heat-sensitive products, such asgelatine, it is highly desirable that the drying air be supplied atconstant conditions of temperature and humidity.

In order to maintain a constant temperature and humidity of the dryingair, it is necessary to insure: (1) that the air-circulation systemremain essentially adiabatic, that is, at constant wetbulb temperature,and (2) that the air-reconditioning step in chamber 1 be carefullycontrolled. These requirements will now'be considered in detail.

(l) The apparatus of the invention, since it involves closed cycles ofboth the air and the hygroscopic liquid, would of necessity remainadiabatic if it were not for the possibility of heat loss or gain in theliquid drawn ofi to the regenerator 20, for heat transfer by radiationbetween the room and the walls of the chambers I and I and connectingducts and piping, and for slight frictional effects. In the preferredoperation of the process, there is little heat loss or gain in theregenerator circuit since the reconcentrated liquor may, by means of theheatexchanger 23, be returned through the pipe 26 at substantially thesame temperature as the diluted liquor withdrawn through the pipe l8, ashereinbefore explained. However, although heat transfer to and from theroom is minimized by covering the apparatus with heat insulation, it canrarely be entirely eliminated. To this end, a control instrumentresponsive to variations in the total heat content of the system, suchas a wet-bulb thermometer 33, is placed in the drying chamber i. Whenthe apparatus is to be run at a temperature above that of itssurroundings. so that the total heat content of the system tends todecrease because of radiation losses, the wet-bulb thermometer 33 isconnected operatively to the valve 25 controlling the heat exchan erby-pass 24, as illustrated: the valve 28 controlling the cooler 21 iskept closed. Then when the total heat of the system fails, thethermometer 33 drops slightly in temperature, opening the valve 25, andallowing some of the reconcentrated .liquor from the evaporator 20 toby-pass the heat-exchanger 23. Under this condition the reconcentratedliquor returns to the circulatory system through the pipe 26 at a highertemperature than the diluted liquor leaving through the line l8. Heat isaccordingly added to the liquid circulatory system and is, of course,transferred to the air-circulatory system, since the two are in contactin the chamber 1, so that the total heat content of the entire apparatusrises until the desired value is reached, at which time the wet-bulbthermometer operates to close the valve 25. On the other hand, when thedrying apparatusis to be run at a temperature lower than that of itssurroundings so that the total heat of the system tends to increase byradiation into the chambers l and I, the wet-bulb therof the systemrises, the thermometer 33 operates to open the valve 28, cooling thereconcentrated liquor returning to the system in the pipe 25 to atemperature below that of the diluted liquor leaving the pipe l8. Heatis extracted from the entire system until the desired heat content isattained, when the thermometer 33 closes the valves 23.

In an alternative structure of the apparatus, not illustrated, theheat-exchanger 23 may be omitted, in which case the small heat-exchanger2! is connected so as to be either a heater or cooler in response to thedemands of the wetbulb thermometer 33. In fact, in the broadest sense ofthe invention, the wet-bulb thermometer 33 may be used to controlmake-up heating or cooling means located anywhere within the air orhygroscopic liquid circulating systems, since these systems areinter-communicating and are otherwise adiabatic. It is thus evident thatthe wet-bulb thermometer 33 is essentially a means responsive tovariations in the total heat content of the air and hygroscopic liquidcirculatory systems and adapted to actuate means for increasing ordecreasing the total heat content of the two systems. v

(2) As here'inbefore noted, if the drier is to be operated on air ofconstant temperature and humidity, not only must the system remain atsubstantially constant total heat, but in addition the airreconditioning step in the'chamber i must be. carefully controlled so asto maintain constant conditions. That is, the air-hygroscopic liquidcontacting step must be operated so that the efiectiveness thereof, 1.e. the quantity of moisture removed from the total quantity of air aftereach passage through the chamber i, is such that the amount of waterabsorbed by the hygroscopic solution equals the amount of water givenoff by the material being dried. This effectiveness depends upon therate of flow of air through the chamber I, the'rate of flow ofhygroscopic solution from the reservoir ll into the sump i2, and the.concentration of the solution. In the apparatus iilustratedjthe rate ofair flow remains constant as long as the damper 3 is closed and the fan3 operates at constant speed; the concentration of the hygroscopicsolution is held constant by the density-responsive plummet 3!. Theeffectiveness of the reconditioning step is then dependent only on therote of flow of hygroscopic liquid. Accordingly, to provide automaticcontrol, the valve I6 governing this flow is connected operatively to adry-bulb thermometer or thermostat 32, placed in the drying chamber I,as shown, and set at the drying temperature desired. Then, when thetemperature of the air entering the drier I changes due, say, to avariation in the rate of moisture evolution of the material being dried,the thermostat 32 operates to open or close the valve l6, thus varyingthe effectiveness of the air-reconditioning in the chamber i to justsuch an extent that the temperature of the reconditioned air returns tothe desired value.

Instead of connecting the thermometer 32 to the valve it as shown, thesame temperature control may be effected by connecting it to any otherof the means for varying the effectiveness of the air-reconditioningstep in chamber 1. Thus, if

the valve i6 and density-device 23 be kept at aconstant setting, thethermostat 32 may be connected so as to vary the speed of the fan 3, orto control the quantity of air passing through the chamber 1 byautomatically controlling the relative positions of the dampers 6 and8., Again, if the valve I6,fan 3, and dampers 6 and 8 are at a constantsetting, the thermostat 32 may be connected to control the concentrationof the I hygroscopic liquid, e. g. varying the addition of heat to theevaporator 20 by means not illustrated, or by adding additionalquantities of one of the components to the solution. Inthese in stancesthe density control 29 and auxiliary parts are omitted. In any of thesemodifications, the thermostat 32 may be considered as a means responsiveto variations in temperature in the drying chamber I and adapted tocontrol the efiectiveness of the air-reconditioning operation in thechamber 1.

As will be evident from the foregoing, the ultimate function of thethermostat 32 and the wetbulb thermometer 33 is to control thetemperature and humidity of the air entering the drier l. The wet-bulbthermometer 33 has hereinbefore been discussed as means responsive tothe total heat content of the system, i. e. wet-bulb temperature.However, as long as the dry-bulbtemperature is under control, the totalheat content and humidity are interdependent, so that the wet-bulbthermometer 33 may equally well be considered as means responsive to thehumidity of the air. Thus, it has been found that any other humidityormoisture-responsive instrument, e. g. any hygrostat, will operateequally as wel1 as the wet-bulb thermometer 33. Indeed, since in theapparatus the'temperature and humidity are both under independentcontrol in an essentially adiabatic system, it has been found possibleto interchange the connections of the thermostat 32 and humidity device33 without greatly affecting the operation of the apparatus. Theseinterchanged controls are not, however, quite as rapidly responsive ascontrols connected as illustrated, and hunting may at times occur.

The operation of the apparatus as particularly described above has beenwith" reference, first, to adiabatic drying in the absence of anycareful control of temperature or humidity, and, then, as is preferable,with temperature and humidity carefully maintained constant by suitablecon trols. However, with certain materials it is desirable to changegradually the state of the air entering the chamber I during the courseof the drying operation. This can be accomplished according to thepresent invention by constantly or intermittently changing the settingof the thermostat 32 and hygrostat 33 in any prescribed manner, theoperation being otherwise as described.

The apparatus of the invention may be .operated using any hygroscopicliquid or solution, as sulfuric acid, glycerine, aqueous solutions ofcalcium and lithium halides, and the like. The

rocess is notlimited to the removal of water by air drying, but isequally applicable to the removal of any volatile liquid from materialsmoist therewith, using any inert gas as the circulating medium, and anysuitable absorbent liquid capable of being reconcentrated.

It will be appreciated that in the invention the drying operation in thechamber l, the air-reconditioning operation in the chamber I, and thecirculating system for the hygroscopic liquid all are maintained undersubstantially adiabatic conditions. -It is entirely unnecessary toprovide external means for heating the air going into the drying chamberor to discard any of the air leaving such chamber, as is necessary inmost drying methods hitherto known. The only heat requirement of theprocess, besides that for slight radiation losses, if any, is theheatrequired to reconcentrate the hygroscopic solution. This quantity ofheat may, if multiple-eflect evaporation of the hygroscopic solution isemployed,'be markedly less than that theoretically required to dry thematerial charged. The heat economy and efliciency thus obtained are veryhigh.

This application is a division of my 'co-pending application Serial No.286,536, flled July 26, 1939, which is a continuation-in-part of myapplication Serial No. 245,401, filed December 17, 1938, which latter isin turn a continuation-in-part of my prior application Serial No.13,968, filed March 20, 1935.

Other modes of applying the principle of the invention may be employedinstead of those explained, change being made as regards the detailsdisclosed, provided any of the stepsor means stated in any of thefollowing claims or the equivalent of such stated steps or means beemployed.

I claim:

1. A system. for drying materials comprising: a closed thermallyinsulated circulatory system for a body of air comprising a drying zoneincluding means for bringing the air into contact with materials to bedried, a reconditioning zone including means for contacting the air witha hygroscopic liquid so as to dehumidify and warm the air, andcirculating means for passing the air successively through said dryingand reconditioning zones; a closed thermally insulated liquidcirculatory system for the hygroscopic liquid including means forcirculating the liquid through the aforesaid air-reconditioning zone; aliquid concentrating system including means for withdrawing the liquidfrom the liquid circulation system, means for concentrating theliquidwithdrawn, heat-exchange means for controlling the temperature of theconcentrated liquid, and means for returning the concentrated liquid tothe liquid circulatory system; and a control system including meansresponsive to both the humidity and the temperature of the air entering-the aforesaid drying zone, the said means controlling both theaforesaid heat-exchange means and the quantity of moisture removed fromthe air during the passage through the air-reconditioning zone.

2. In a drying system comprising an air-flow passage, in combination: agas and liquid contact zone in said passage; means for recirculating ahygroscopic liquid through said zone and including a sump; a regeneratorfor concentrating the hygroscopic liquid; circulating means for passingliquid from said sump to said-regenerator and after concentration backto said sump; heatexchange means for passing the liquid flowing to theregenerator in heat exchange relation with the liquid flowing from theregenerator; means responsive to the total heat content of the gaspassing from said contact zone; and means controlled by the last-namedmeans for controlling the heat-exchange means.

3. In a drying system, comprising an air-flow passage, in combination: agas and liquid contact zone in said passage; means for recirculating ahygroscopic liquid through said zone and including a sump; a regeneratorfor concentrating the hygroscopic liquid; circulating means for passingliquid from said sump to said regenerator and after concentration backto said sump; heatexchange means for passing the liquid flowing to theregenerator in heat-exchange relation with the liquid flowing from theregenerator; means responsive to the wet-bulb temperature of the gaspassing from said contact zone; and means controlled by said last namedmeans for controlling the heat-exchange means.

4. In a drying system comprising an air-flow passage, in combination: agas and liquid contact zone in said passage; means for recirculating ahygroscopic liquid through said zone and including a sump; a regeneratorfor concentrating the hygroscopic liquid; circulating means for passingliquid -1rom said sump to said regenerator and after concentration backto said sump; heat-exchange means for passing the liquid flowing to theregenerator in heat-exchange relation with the liquid flowing from theregenerator; means responsive to the wet-bulb temperature of the gaspassing from said contact zone; and means controlled by said last-namedmeans for lay-passing said liquid around said heat-exchange means.

5. In a drying system comprising an air-flow passage, in combination: agas and liquid contact zone in said passage; means for recirculating ahygroscopic liquid through said zone and including a sump; a regeneratorfor concentrating the hygroscopic liquid; circulating means for passingliquid from said sump to said regenerator and after concentration backto said sump; heatexchange means for controlling the temperature of theliquid flowing back to said sump; means responsive to the humidity ofthe gas passing from said contact zone for controlling the heatexchangemeans; and means responsive to the temperature of the gas passing fromsaid contact zone for controlling the quantity of moisture removed fromthe air during passage through the contact zone.

6. In a drying system comprising an air-flow passage, in combination: agas and liquid contact zone in said passage; means for recirculating ahygroscopic liquid through said zone and including a sump; a regeneratorfor concentrating the hygroscopic liquid; circulating means for passingliquid from said sump to said regenerator and after concentration backto said sump; heatexchange means for controlling the temperature oftheliquid flowing back to said sump; means responsive to the humidity ofthe gas passing from said contact zone for controlling the heatexchangemeans; and means responsive to the temperature of the gas passing fromsaid contact zone for controlling the rate of recirculation of thehygroscopic liquid through the said zone.

7. In a drying system comprising an air-flow passage, in combination: agas and liquid contact zone in said passage; means for recirculating ahygroscopic liquid through said zone and including a sump; a regeneratorfor concentrating the hygroscopic liquid, circulating means for passingliquid from said sump to said regenerator and after concentration backto said sump; heat exchange means for controlling the temperature of theliquid flowing back to said sump; means responsive to the humidity ofthe gas passing from said contact zone for controlling the heat-exchangemeans; means responsive to the temperature of the gas passing from saidcontact zone for controlling the rate of recirculation oi. thehygroscopic liquid through the said zone; and meansresponsive to theconcentration 01 the liquid in said sump and adapted to control saidregenerator.

8. In a drying system comprising an air-flow passage, in combination: agas and liquid contact zone in said passage; means for recirculating ahygroscopic liquid through said zone and including a sump; 'aregenerator for concentrating the hygroscopic liquid; circulating meansfor passing liquid from said sump to said regenerator and afterconcentration back to said sump; a first heat-exchange means for passingthe liquid flowing to the regenerator in heat-exchange relation with theliquid flowing from the regenerator; a valved by-pass for passing theliquid flowing from the regenerator around said first heat exchangemeans; a second heat-exchange means for controlling the temperature ofthe liquid flowing back to said sump; means responsive to the humidityof the gas passing from said contact zone for controlling the valvedlay-pass and also for controlling said second heat-exchange means; meansresponsive to the temperature of the gas passing from said contact zonefor controlling the rate of recirculation of the hygroscopic liquidthrough the said zone; and means responsive to the concentration of theliquid in said sump for con- 10 trolling said regenerator.

FRANCIS R. BICH OWSKY.

